A. Field of the Invention
The invention relates to a marine propulsion set with a pod installable under the hull of a ship.
B. Description of Related Art
Generally, conventional marine propulsion sets of the pivotally mounted pod type are not designed to work in the wake of the ship and indeed have a support strut that is long enough for its propeller to be situated outside the boundary layer of the wake. Such conventional pivotally mounted pod type propulsion sets are generally voluminous at least due to the large amount of space necessary between the hull of the ship and the propeller of the set. Furthermore, such propulsion sets are generally subjected to vibration and cavitation phenomena, cavitation being particularly present when the propulsion set is slewing. Cavitation is a phenomenon that releases explosive bubbles of water vapor at the ends of the blades of a propeller. In marine hydrodynamics, cavitation degrades the performance of propulsive systems, induces vibration, causes erosion of the rotary portions, and radiates noise that degrades the acoustic discreetness of the ship.
From certain prior art documents and in particular from Patent Document EP 1 270 404, it is known that there exists a propulsion set as defined above in which a propeller of an auxiliary propulsion unit of the compact pivotally mounted pod type is situated at the aft end of the pod. Furthermore, that propeller is designed to work in the wake of another propeller or “main screw” which is mounted on a fixed shaft disposed under the hull of the ship. The main screw is designed to deliver the majority of the propulsive power, e.g. by means of a diesel engine installed in the ship, whereas the auxiliary propeller or “screw” of the propulsion pod is designed to deliver either additional propulsive power or directional power if the propulsion pod is pivoted for steering the ship. In the versions with an arrangement of fins around the pod, that arrangement of fins is situated either on the forward portion of the pod or further aft, but no further aft than the central portion of the support strut. The purpose of the fins is to improve the propulsive efficiency by recovering the axial component of the rotary energy from the vortex generated by the main screw, and they must therefore be relatively close to the main screw. It is possible to make provision for the fins to be inclined to a small extent relative to the axis of the pod in order to increase the energy recovery.
Although such a pivotally mounted propulsion pod is particularly compact, the overall propulsion set including the main screw remains voluminous and requires a relatively deep draught under the hull as do conventional pivotally mounted pod type propulsion sets.